Sunday 11 April 2010 14.54 EDT
First published on Sunday 11 April 2010 14.54 EDT

The Conservative party today kept up the pressure on the government over its plans to increase national insurance, announcing the names of a further 23 business leaders who had signed a letter opposing the tax rise, taking the number to more than 100.

Among the latest additions to the business leaders speaking out against the proposals are the former Labour donor and millionaire rice importer Moni Varma and the head of betting chain William Hill, Ralph Topping.

With the stand-off between business and Labour intensifying, the new names bring the total to 104 business chiefs, who between them employ more than one million people.

The signatories to the Conservative co-ordinated letter, first published on 1 April in the Telegraph, also include Sir Stuart Rose, the executive chairman of Marks & Spencer, and Sir Stelios Haji-Ioannou, the founder of easyGroup.

The letter argues that the government's planned one percentage point rise in national insurance contributions next year will put jobs and the recovery at risk and says the potential gains should instead be found through "cutting waste". Labour wants to raise NICs for people on more than £20,000 from April next year, but the Tories have pledged to cancel most of the planned rise.

Explaining his support, Topping said: "William Hill is a major employer, with almost 16,000 employees in the UK.

"We have worked hard to maintain jobs in difficult economic circumstances, including a pay freeze that has lasted nearly 30 months in a business that faces significant challenges in the UK.

"Such tough measures aimed to avoid the need to lay people off or close shops, which is exactly what we would be facing if the level of taxation increased."

Previous signatories to the Tory letter include Richard Caring, a former Labour donor and chairman of Soho House; Simon Fox, the chief executive of the HMV Group; the Diageo chief executive, Paul Walsh; and Brent Hoberman, the co-founder of lastminute.com and a member of Gordon Brown's business council.

The Liberal Democrat Treasury spokesman, Vince Cable, last week said businessmen on inflated salaries wading into to the national insurance debate were "being used" by the Conservatives.

Cable – the public's current choice for chancellor, according to polls – also said the Tories were guilty of "barefaced cheek" in the ongoing row. "I just find it utterly nauseating – all these chairmen and chief executives of FTSE companies being paid 100 times the pay of their average employees lecturing us on how we should run the country," he said in an interview with the Guardian.

The Conservative leader, David Cameron, said his party's planned efficiency savings of £12bn this year were "perfectly doable" and could be used to prevent the NICs rise. But the chancellor, Alistair Darling, hit back, suggesting the savings were a mirage.

Who backs Cameron?

•Moni Varma, founder and chairman, Veetee. India-born Varma founded Veetee Rice in 1987 and built it into one of the UK's largest rice suppliers

•Peter Jones, chief executive, Associated British Ports. Took the helm at the ports operator in 2007. His company operates 21 ports around the UK and handles around a quarter of the country's seaborne trade.

•Toni Mascolo, chief executive, Toni & Guy. Italian-born Mascolo opened his first salon with his brother Gaetano, or "Guy", in Clapham, south London in 1963. His company now has more than 400 salons worldwide.

•John Nelson, chairman, Hammerson property group. The former boss of investment bank CSFB Europe is also deputy chairman at retail group Kingfisher, the owner of B&Q.